Biomedical Engineering Reference
In-Depth Information
LUMEN OF GUT
epithelial cell migration
from “birth” at the bottom
of the crypt to loss at the
top of the villus
(transit time is
3-5 days)
epithelial
cells
villus (no cell division)
cross-section
of villus
villus
absorptive
brush-border
cells
crypt
loose
connective
tissue
cross-section
of crypt
mucus-
secreting
goblet cells
nondividing
differentiated
cells
direction of cell
movement
rapidly dividing
cells (cycle time =
11 hours)
crypt
slowly dividing stem
cells (cycle time >
24 hours)
nondividing
differentiated
cells
100
μ
m
(a)
(b)
FIGURE 6.10
Villi in the small intestine. (a) The villi and the crypt indicating the mitotic state of the cells in
various locations. (b) Rows of villi of epithelial intestinal cells (the diameter of a villi is about 80 mm).
From [1].
takes place in set locations within the crypt. Once the cells are mature, they migrate to
the outer edge of the crypt and then move over a period of about five days from the base
of the villus to the top, where they die and slough off. During this passage, the cells carry
out their organ-specific function as mature
parenchymal cells
. They function in the absorp-
tion and digestion of nutrients that come from the lumen of the gut. Toward the bottom of
the crypt is a ring of “slowly dividing” determined stem cells. The number of stem cells per
crypt is about 20. After division, the daughter cell moves up the crypt, where it becomes a
rapidly cycling progenitor cell, with a cycling time on the order of 12 hours. The cells that
are produced move up the crypt and differentiate. Once they leave the crypt, they are
mature and enter the base of the villi.
Skin
Human skin has two principal cell layers, an epidermis and dermis, separated by a form
of extracellular matrix called a basal lamina (or basement membrane) (see Figure 6.4). In
this extracellular matrix, collagen VII is an important component. The two layers undulate
with respect to each other, and the undulations produce deep pockets (distant from the skin
surface) and other pockets that are shallower. The pattern of the relationship between layers
is unique for each type of skin (e.g., that on the trunk of the body versus that in the palms of
